In the international market, there are many well-established large-scale micro-Raman spectrometer used for research, all of which are designed with conventional optical elements and large structures of more than one meter in length. The use of surface-enhanced Raman scattering (SERS)substrates allows for ultra-high-sensitivity SERS-micro-Raman spectroscopy testing function. Such a device has high sensitivity, good performance, high cost, and large size but cannot be widely and portably used.
In addition, in the international market, although there are many models of portable small Raman spectrometers, which usually use fiber Raman probe disclosed in U.S. Pat. No. 5,122,127. The optical fiber Raman probe has a numerical aperture NA of 0.25, which is corresponding to a small collecting aperture angle Ω, where Ω is about 0.25sr, and the sensitivity is applicable to only analysis of samples with higher abundance and higher concentration, leading to restriction on popularization and application. There is urgent demand on upgrading and developing portable probes and small Raman spectrometers with ultra-high sensitivity.
At present, due to that: the poor coherence of the semiconductor laser beams used by many optical fiber Raman probes to excite samples, the coherence of the beams for sample excitation is destroyed via multimode optical fiber transmission, some compact Raman probes and spectrometers using objective NA smaller even with aspherical objective NA≈0.5 to focus the laser beam to excite the sample, the quality of the objective and the laser beam do not meet the requirements and other factors, make it difficult to effectively use the SERS substrate to achieve ultra-high sensitivity.
At the present stage, with the condition of the existing lasers and Charge Coupled Device (CCD) photo-detectors, to obtain SERS high enhancement, there are the following three most basic requirements: the need to ensure that the laser beam for exciting the samples has a good coherence; appropriate focus spot; and optical systems on the order of 2-10 microns and collecting Raman scattering with large aperture angles. However, all the current portable small Raman probes and spectrometers cannot simultaneously meet the three basic requirements of high-sensitivity SERS. Therefore, Raman spectroscopy has low detection sensitivity and cannot be applied to trace analysis on ultra-low concentration samples.